1. Field of the Invention
The present invention relates to an image forming apparatus and a control method of the image forming apparatus.
2. Description of the Related Art
An image forming apparatus starts conveying a sheet, which has been kept standby at a registration roller pair, after finishing rasterization of image data. The image forming apparatus transfers a developer image corresponding to the rasterized image data, onto the conveyed sheet and fixes the developer image on the sheet, thereby achieving an image formation on the sheet.
Recently, many high speed image forming apparatus have been conveying sheets to a transfer unit and a fixing unit at regular time intervals. In the high speed image forming apparatus, the fixing unit is configured to be continuously heated at a constant temperature such that a temperature decrease caused by a sheet passing through the fixing unit can be compensated by conveyance time intervals of sheets. FIG. 10 illustrates a temperature change of the fixing unit when normal printing of a plurality of pages is performed. In FIG. 10, the temperature of the fixing unit is kept at an optimum temperature for fixing an image onto a sheet by compensating the temperature decrease caused by the sheet passing through the fixing unit with the conveyance time intervals of sheets.
However, when the fixing unit has the above described configuration, if one page takes longer time to process an image, the temperature of the fixing unit may become higher than the optimum temperature. This situation arises when a standby time of the sheet at the registration roller pair becomes longer, therefore, it takes longer time to rasterize the image data, i.e., a fixing device continues to be heated during the rasterization, resulting in increase of a temperature of the fixing device. If the temperature of the fixing device becomes equal to or more than the temperature optimum for fixation processing due to the above described temperature increase, the image forming apparatus is kept in a standby state while a heater of the fixing device is turned off in order to decrease the temperature of the fixing device to the temperature optimum for the fixation processing.
FIG. 11 illustrates a temperature change of the fixing unit when a cycle down occurs in printing a plurality of pages. In FIG. 11, it takes a longer time to fix image data on a third page, resulting in bringing the fixing unit to a temperature over the degree optimum for fixing an image on a sheet (to an abnormal temperature). If the temperature of the fixing unit increases to the abnormal temperature, heating at the fixing unit needs to be once stopped to wait until the temperature of the fixing unit recovers to the optimum temperature. It is called the cycle down. In FIG. 11, the fixation processing of image data is restarted with respect to the image data of the third page after performing the cycle down, i.e., after the temperature of the fixing unit recovers to the optimum temperature.
Japanese Patent Laid-open Publication No. 11-10962 discusses a method for minimizing decrease of an overall throughput in an image forming apparatus making prints by effectively utilizing time even with respect to extremely complicated print data.
In the method discussed in the Japanese Patent Application Laid-Open No. 11-10962, bit map data is created for each of color components when a multicolor printing is performed on a print medium, such as a card or a label, which are conveyed at a constant speed. If the bit map images, which are to be printed onto the print medium, cannot be timely created although the print medium has been conveyed to a position where the printing is to be performed, the printing of the immediately preceding print medium is performed. After finishing the printing, the print medium is reversely conveyed, and printing of the bit map data which could not be timely created is performed. After finishing the printing of that data, the process is returned to a normal direction and the normal processing is recovered.
However, the method in the Japanese Patent Application Laid-Open No. 11-10962 discusses only switchback of a sheet and the temperature increase of the fixing unit cannot be effectively prevented. Further, since the recent printers were basically configured to be high speed printers, it is extremely difficult to carry out the switchback of the sheet while keeping the high speed during print processing.